Combustion of coal, oil, natural gas, industrial waste, domestic waste, peat, etc. produces flue gases that may contain gaseous pollutants, such as sulphur dioxide, hydrofluoric acid, bromine and hydrochloric acid. Gaseous pollutants, such as sulphur dioxide, can also be produced as a residual product in chemical processes, for instance in metallurgical processes. Lately carbon dioxide, CO2, has also been noticed as a gaseous pollutant due to its negative effects on the environment. To avoid emitting large quantities of gaseous pollutants into the atmosphere a gas cleaning device is normally used for separating from the gas, at least partly, the gaseous pollutants before emitting the gas into the atmosphere.
WO 2005/007274, which is assigned to ALSTOM Technology Ltd, describes a method and a device for removing sulphur dioxide from a flue gas. The flue gas is passed upwards through an apertured plate with a flowing layer of an absorption liquid provided thereon. The absorption liquid may contain, for instance, water and one or more of the following substances: lime, limestone, dolomite, sodium hydroxide solution and similar substances, which are suitable for absorption of sulphur dioxide. The absorption efficiency of the flowing layer depends on its thickness. In one embodiment shown in WO 2005/007274, a valve is provided in an outlet zone and is located just below the level of the apertured plate. The purpose of this valve is to control the thickness of the flowing layer of the absorption liquid on the apertured plate. It has been found, however, that this valve is not very effective for controlling the flowing layer and that the flowing layer often becomes quite unstable.